1. Field of the Invention
The present invention relates to methods for capacitive drive excitation of oscillators for sensors for capacitive measurement of physical quantities such as force, acceleration and rotation rate by determination of Coriolis forces. More particularly, this invention pertains to such a method in which a sequence of electrical pulses, tuned to the natural frequency of the oscillator is applied to a drive capacitor connected to the sensor. Preferably, but not exclusively, plate-like oscillators may be employed. The sensors employed are of the type in which frequency-analog-type excitation methods and/or spin preserving measuring principles are employed that accept rotation or pulses, and in which variations in rotation and pulses are picked up during reading.
2. Description of the Prior Art
The Coriolis principle is generally considered as being well known. Coriolis rotation rate sensors currently available on the market employ piezoelectric effects for both the drive and readout systems. A recent design of such a sensor, to which reference will be made below, is described in U.S. Pat. No. 5,959,206.
A general problem of sensors of this type is avoidance of coupling of the drive signal into the readout. Structural variations within tolerances of the oscillating structures or the electrode layers (through which attractive forces that only act exactly orthogonally to the oscillator surface in the ideal case) cause part of the excitation signal to become coupled into the read channel. Since both signals (i.e. the read signal and an unavoidable noise signal due to the above-mentioned inaccuracies) occur with the same frequency, it is necessary to compensate for this error. However, compensation is generally temperature-dependent. A further complication is the fact that the amplitudes to be capacitively read fall in the submicrovolt range, several orders of magnitude beneath those of the exciting signals. This results in stability problems for a zero-point. High attenuations are required to distinguish the drive pulses from the extremely-weak read signals. Due to this, isolation by screening, design symmetry and electronic measures (e.g. separate earth loops for driving and reading (a known principle to avoid the problem that the relatively strong drive signals are induced into the measuring, filtering, amplification and decoding path of the read signals), common-mode rejection in the signal amplification, etc.) are insufficient for more accurate sensors.
Arrangements for sensors of this type have been proposed that employ sensor elements produced micromechanically, preferably from silicon, with capacitive pickoffs (e.g. the above-mentioned United States patent. An electrostatic actuator generally provides the drive. Cross-talk from the excitation to the read electronics poses a problem as well for such known arrangements.
As a solution, an electromagnetic drive has already been proposed in the literature. However, it demands an undesirably large amount of power, greatly increasing the loss of the overall arrangement. Other known solutions employ carrier-frequency methods for reading. Such methods require considerable outlays for the electronics.
Problems posed by known electrostatic drive methods are presented below with reference to FIGS. 4(a) and 4(b). Reference is also made to FIG. 4 of the above-referenced U.S. Pat. No. 5,959,206 which is hereby incorporated by reference.
For the single-sided excitation of a plate-like oscillator 101 (asymmetric arrangement of FIG. 4(a)) having a top electrode 100 in the example presented (or for two-sided excitation via two electrodes 100, 102 per the symmetric arrangement of FIG. 4(b) or FIG. 4 of the above-referenced German patent application), the time profile of the excitation voltages, tuned to the natural frequency of the oscillator 101, and the resulting forces or force pulses are represented by diagrams to the right of FIGS. 4(a) and 4(b) respectively. Arrows of the relevant force/time diagram indicate that, due to the known second-order relationship between drive voltage and resulting forces, only attractive forces are possible between the electrode(s) (indicated only schematically) 101, 100, 102 and the oscillator. In the example of FIG. 4(a) (i.e., in the case of single-sided excitation), it is thus possible to employ only one half-cycle of the exciting voltage for the drive. For two-sided excitation, the time profiles of the exciting voltages and the resulting forces, illustrated as a function of time in FIG. 4(b), show that the oscillator 101 is excited by the time sequence of attractive forces from the two electrodes 100, 102.
In both cases, the pulsed excitation of the oscillator 101 requires the use of relatively high voltages. Portions of such voltages are unavoidably coupled to the read channel electrostatically or by DC coupling.
In order to measure rotation rates within a range of a few degrees per second, the read signals are, as mentioned, many orders of magnitude less than the oscillator excitation signals (voltage pulses). Although coupling to the read signal may be kept small through special layout of the conductors, an alternative in the range of 100 dB or more is very difficult to achieve.
It is therefore an object of the present invention to provide a method for capacitive drive excitation of (preferably plate-like) oscillators of sensors for measurement of rotation rates in which the above-mentioned problem of coupling from the excitation signal to the read channel may be reliably avoided.
The present invention addresses the foregoing by providing, in a first aspect, a method for capacitive drive excitation of an oscillator for capacitive measurement of physical quantities such as force, acceleration and/or rotation rate by determination of Coriolis forces. Such method includes the step of applying a sequence of electrical pulses, tuned to the natural frequency of the oscillator, to a drive capacitor connected to the oscillator. High-frequency pulse packets of constant-amplitude voltage are employed for the drive excitation.
An applied alternating voltage of the pulse packets has a substantially higher frequency than the natural frequency of the oscillator(s), and is preferably free of DC and/or low-frequency voltage components. By adding or subtracting one or more periods of a square-wave voltage in the individual HF pulse packets, it is possible to vary the pulse width of the respective pulse packets, or the pulse phase, using processors to control oscillator excitation.
In a second aspect, the invention provides a method for capacitive drive excitation of an oscillator in a sensor for capacitive measurement of physical quantities such as force, acceleration and/or rotation rate by the determination of Coriolis forces. Such method includes the step of applying a sequence of out-of-phase electrical pulse pairs tuned to the natural frequency of the oscillator to an arrangement of at least two excitation electrodes which is pairwise symmetrical with respect to the oscillation axis. The same constant-amplitude HF voltage is applied to the pair of excitation electrodes. The frequency of the voltage is substantially higher than the natural frequency of the oscillator. The pulse pairs are produced out-of-phase as HF pulse packets by controlled shifting of the phase of the HF voltage applied to one of the ex citation electrodes with respect to that applied to the other of the excitation electrode pair to produce a particular resultant force of attraction on the oscillator.
The latter aspect thus provides for application of the same alternating voltage to the two electrodes of the oscillator electrode pair. These voltages (of the same frequency) applied to the exciter electrodes are then shifted relative to one another in phase. If the excitation voltages are in-phase, then there will be no force component which excites oscillation. If the phase angles differ, there will be a resultant electrical attraction force.
This alternative has t he advantage that the loading conditions for the supply to the oscillation excitation are constant. This is especially the case when the phase angles of the exciter signals are shifted out-of-phase. A further advantage that results is comparatively good isolation of the signal frequency and noise contributions of the electrical drive. The supply is subjected to a high exciter frequency with high-frequency noise contributions filtered out.
According to an advantageous variant of the invention, the phase may be shifted either in discrete steps or continuously.
The preceding and other features and advantages of this invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention. Like numerals refer to like features throughout both the drawing figures and the written description.